Polycrystalline Silicon Structures Research Articles

The Effects of Structure Thickness, Air Gap Thickness and Silicon Type on the Performance of a Horizontal Electrothermal MEMS Microgripper

The ongoing development of microelectromechanical systems (MEMS) over the past decades has made possible the achievement of high-precision micromanipulation within the micromanufacturing, microassembly and biomedical fields. This paper presents different design variants of a horizontal electrothermally actuated MEMS microgripper that are developed as microsystems to micromanipulate and study the deformability properties of human red blood cells (RBCs). The presented microgripper design variants are all based on the U-shape ‘hot and cold arm’ actuator configuration, and are fabricated using the commercially available Multi-User MEMS Processes (MUMPs®) that are produced by MEMSCAP, Inc. (Durham, NC, USA) and that include both surface micromachined (PolyMUMPs™) and silicon-on-insulator (SOIMUMPs™) MEMS fabrication technologies. The studied microgripper design variants have the same in-plane geometry, with their main differences arising from the thickness of the fabricated structures, the consequent air gap separation between the structure and the substrate surface, as well as the intrinsic nature of the silicon material used. These factors are all inherent characteristics of the specific fabrication technologies used. PolyMUMPs™ utilises polycrystalline silicon structures that are composed of two free-standing, independently stackable structural layers, enabling the user to achieve structure thicknesses of 1.5 μm, 2 μm and 3.5 μm, respectively, whereas SOIMUMPs™ utilises a 25 μm thick single crystal silicon structure having only one free-standing structural layer. The microgripper design variants are presented and compared in this work to investigate the effect of their differences on the temperature distribution and the achieved end-effector displacement. These design variants were analytically studied, as well as numerically modelled using finite element analysis where coupled electrothermomechanical simulations were carried out in CoventorWare® (Version 10, Coventor, Inc., Cary, NC, USA). Experimental results for the microgrippers’ actuation under atmospheric pressure were obtained via optical microscopy studies for the PolyMUMPs™ structures, and they were found to be conforming with the predictions of the analytical and numerical models. The focus of this work is to identify which one of the studied design variants best optimises the microgripper’s electrothermomechanical performance in terms of a sufficient lateral tip displacement, minimum out-of-plane displacement at the arm tips and good heat transfer to limit the temperature at the cell gripping zone, as required for the deformability study of RBCs.

Study on Dynamic Model of Polycrystalline Silicon Photocell

The silicon photocell is a common solar cell, and the single crystal silicon photocell is the best among them. But the polycrystalline silicon photocell has many advantages such as simple manufacturing process and low cost. To deeper understand the working principle of polycrystalline crystal silicon photocell and the factors that affect solar cell performance, the equivalent circuit of photocell are simulated by using Multisim Software and AMPS-1D software in this paper. On this basis, we made some experimental simulations, and analyzed the influence of the thickness, doping concentration, local density of states, the potential barrier of before and after contact surface, the HT-EBL and ET-HBL structure of polycrystalline silicon on its open circuit voltage, short circuit current, fill factor and conversion efficiency. The results show that the thickness, doping concentration, local density of states, the potential barrier of before and after contact surface, the HT-EBL and ET-HBL structure of polycrystalline silicon photocell will affect on the open circuit voltage, short circuit current, filling factor and conversion efficiency. The best polycrystalline crystal silicon photocell conversion efficiency could reach 12.384% after adding the before and after contact layer.

Grain size and doping effect on structure and electromechanical properties of polycrystalline silicon for MEMS applications

Grain size effect on microhardness and Young modulus of polysilicon layers was obtained by nanoindentation experiments: 14.1-16.3 GPa and 240-300 GPa, respectively, for the particle-diameter of 117-430 nm. The range of grain size correlated to mechanical properties exceed theoretical values was defined. The doping effect was considered. The grain size range of PECVD polysilicon with aspects of hardening and suitable conductivity is presented with the objective to formation of micro- and nanomechanical devices.

Design, Simulation, and Fabrication of Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET) With New Termination Structure

In this paper, we introduce a new type of termination structure utilizing semi-insulating polycrystalline silicon (SIPOS) structures in conjunction with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> junction extension in order to reduce the area of termination device structure and increase the breakdown voltage. In SIPOS structures, one high-resistance layer is deposited between electrodes on two terminals such that the voltage between two electrodes is linearly distributed instead of the original nonlinear distribution. Compared to a traditional termination structure device (with a length of 180 μm and a breakdown voltage at 570 V), the optimal design of the new type of termination structure will lead to an increase of 40 V in the breakdown voltage and a 22.2% area reduction. From the perspective of device fabrication, it has been proven that the new type of termination structure indeed consumes less area while simultaneously realizing an enhanced device breakdown voltage as high as 710 V and a specific on-resistance at 137 mΩ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is consistent to the simulation results.

Structural features of high purity silicon prepared by electron-beam refining of metallurgical-grade silicon

A study of the structure of high purity polycrystalline silicon, prepared by electron beam refining of metallurgical-grade silicon in a vacuum is provided. Results of the study point to sensitivity of impurity particle growth and dimensions to metallurgical-grade silicon refining method (vacuum or oxidation), in contrast to crystallite dimensions. There is also almost total absence of impurity segregation towards grain boundaries during silicon crystallization from a melt.

In Situ Tuning of Focused-Ion-Beam Defined Nanomechanical Resonators Using Joule Heating

Nanomechanical resonators have a huge potential for a variety of applications, including high-resolution mass sensing. In this paper, we demonstrate a novel rapid prototyping method for fabricating nanoelectromechanical systems using focused-ion-beam milling as well as in situ electromechanical characterization using a transmission electron microscope. Nanomechanical resonators were cut out of thin membrane chips, which have been prefabricated using standard cleanroom processing. We have demonstrated the fabrication of double-clamped beams with feature sizes down to 200 nm using a fabrication time of 30 min per device. Afterwards, the dynamic and structural properties of a double-clamped beam were measured after subsequent Joule heating events in order to ascertain the dependence of the internal structure on the Q-factor and resonant frequency of the device. It was observed that a change from amorphous to polycrystalline silicon structure significantly increased the resonant frequency as well as the Q-factor of the nanomechanical resonator. Aside from allowing detailed studies of the correlation between internal structure and nanomechanical behavior on an individual rather than a statistical basis, the combination of a short turnaround time and in situ nonlithographic tuning of the properties provide a flexible approach to the development and prototyping of nanomechanical devices.

Effect of Rapid Thermal Annealing Condition on the Structure and Conductivity Properties of Polycrystalline Silicon Films on Glass

The effect of rapid thermal annealing (RTA) temperature (700~1200 °C) and time (1~8 min) on the structure and conductivity properties of polycrystalline silicon (Si) films on glass, grown by RTA crystallization of magnetron sputtering (MS) deposited amorphous Si (a-Si) films, was investigated by means of X-ray diffraction (XRD) and UV reflectance. It was observed the critical temperature for crystallizing a-Si films was ~750 °C and ~700 °C based on XRD and reflectance measurements, respectively. As soon as RTA temperature reached and exceeded the critical value, the structural property of polycrystalline Si films increased with RTA temperature or time. The above results were related to the thermal and photon effects induced by RTA. Moreover, it was revealed that the resistivity of polycrystalline Si films decreased with RTA temperature, however, even the resistivity of the polycrystalline Si films annealed at 1200 °C was 2 orders of magnitude higher than that of Si target, attributed to the carrier scattering by grain boundaries or defects. The polycrystalline Si films on glass fabricated by MS deposition combined with RTA crystallization may endow them with great application potentials in Si thin-film solar cells.

Self-Aligned Metal Double-Gate Low-Temperature Polycrystalline Silicon Thin-Film Transistors on Glass Substrate Using Back-Surface Exposure

Improving the device structure and quality of polycrystalline silicon (poly-Si) films is essential for enhancing the performance of poly-Si thin-film transistors (TFTs). A multigate structure can potentially be used to realize ideal poly-Si TFTs. We combined a self-aligned metal double-gate structure (i.e., a structure with metal top and bottom gates) with a large-grained poly-Si film. Then, we fabricated high-performance n-channel (n-ch) and p-channel (p-ch) low-temperature poly-Si TFTs on a glass substrate at 550 °C. The nominal field-effect mobilities of the n-ch and p-ch TFTs were 638 and 156 cm2 V-1 s-1, respectively, and the s-values of both the TFTs were nearly 130 mV/dec.

Effects of layers and vias on continuous-wave laser heating and damage of surface-micromachined structures

The response of microsystem components to laser irradiation is relevant to processes and applications such as laser processing, optical diagnostics, and optical microelectromechanical systems (MEMS) device design and performance. The dimensions of MEMS, which are on the order of several micrometers, are on the same order as infrared laser wavelengths, which results in interference phenomena when the parts are partially transparent. Four polycrystalline silicon structures were designed and irradiated with 532 and 808 nm continuous wave laser light to determine the effects of layers, air gaps, and the presence of a substrate via on the threshold laser power for damage. The presence of a substrate via resulted in lower damage thresholds, and a single-layer structure had the highest damage threshold for structures irradiated with infrared light. Structures irradiated with visible wavelength light exhibited less sensitivity to the underlying via. Optical interference calculations are carried out to evaluate the absorptance of these structures as a feasible explanation for the observed results.

OS1215 応力集中部を有する微小多結晶Si構造体における寸法効果(ナノ・マイクロからの視点からの変形と破壊の力学(2),オーガナイズドセッション)

OS1215 応力集中部を有する微小多結晶Si構造体における寸法効果(ナノ・マイクロからの視点からの変形と破壊の力学(2),オーガナイズドセッション)

Strength Reliability of Micro Polycrystalline Silicon Structure

In order to evaluate strength reliability of micron size polycrystalline silicon (poly-Si) structure, bending tests of cantilever beam and Weibull analysis are performed. Recently, the importance of microelectromechanical systems (MEMS) in society is increasing, and the number of production is also increasing. The MEMS devices, which contain mechanical movement, have to maintain their reliability in face of external shock, thermal stress and residual stress from manufacturing processes. In greeting the mass production era of the MEMS, in case the material strength design of MEMS is performed, required strength data is not average value but the variation, especially minimum value of the material. Micron size poly-Si structure is widely employed in the MEMS such as microsensor, switching device and so on. Then, in order to evaluate strength reliability of micron size poly-Si structure, tests and analysis are performed. The specimen is made by chemical vapor deposition (CVD) process and thickness is 3.5, 6.4 and 8.3 micrometer and the specimen has notch. The test specimen used for the test changed characteristics of (1) film thickness (2) stress concentration, and investigation about the influence each effects of the variation in a bending strength are discussed.

Suppression of Self-Heating in Low-Temperature Polycrystalline Silicon Thin-Film Transitors

We investigated the structure of low-temperature polycrystalline silicon (poly-Si) thin-film transistors (TFTs) focusing on their immunity against thermal degradation. Their operating temperature was simply dependent on input power and independent of bias voltage, such as drain or gate bias voltage. As for the structures, self-heating was suppressed by increasing the number of splitting gates and the interval between poly-Si layers owing to effective heat diffusion along the gate width. For multi gate-type TFTs, increasing the number of splitting gates was effective in suppressing self-heating; however, increasing the interval between gates was not effective. We proposed a new offset-type structure. Using this new structure, we were able to demonstrate the effective suppression of degradation caused by self-heating.

Efficient 1.54μm light emitting diode with nanometer thick polycrystalline Si anode and organic sandwich structure

This letter reports that the 1.54μm electroluminescence efficiency of the organic light emitting diode (OLED) with a structure of nanometer thick polycrystalline silicon (NTPS)/NPB/ErQ/AlQ/Al is two orders of magnitude higher than that of the OLED with a structure of thick crystalline silicon/NPB/ErQ/Al, which is similar to the OLED reported in literature [Curry et al., Appl. Phys. Lett. 77, 2271 (2000)]. Such an improvement is mainly attributed to the fact that hole injection is controlled by NTPS anode and holes are blocked by AlQ to match electron injection, and a higher light out coupling as well.

Laser-based measurement of elastic and mechanical properties of layered polycrystalline silicon structures with projection masks

The Young’s modulus, density, and thickness of a 20.3μm polycrystalline silicon layer deposited on a silicon wafer covered with a ∼2.5-μm-thick silicon-oxide interface layer were measured using projection masks to generate surface acoustic waves (SAWs) with higher harmonics approaching 600MHz. The propagating SAW train was detected with a laser probe-beam-deflection setup. The characteristic strongly nonlinear dispersion effect allowed the simultaneous extraction of several unknown film properties. The dispersion was described theoretically by the boundary element method model. A Young’s modulus of 152GPa, a density of 2.25g∕cm3, and a film thickness of 20.3μm were determined for the polycrystalline silicon film.

Doping Effect on the Structure of Polycrystalline Silicon Films Grown via Silane Pyrolysis

X-ray diffraction, transmission and scanning electron microscopy, and secondary-ion mass spectrometry data demonstrate that the grain size, structure, and composition of polycrystalline silicon (poly-Si) films grown using hydride pyrolysis depend on the GeH4 : SiH4 volume ratio in the gas mixture, dopant (boron or phosphorus) concentration, substrate temperature, and heat-treatment temperature. The films grown at substrate temperatures below 600°C are amorphous, and those deposited at 600°C are amorphous-crystalline. Textured poly-Si films are only obtained at 620°C or higher substrate temperatures. Doping with germanium, an isovalent impurity, improves the structural perfection of poly-Si films.

Pulsed laser repair of adhered surface-micromachined polycrystalline silicon cantilevers

A technique for repairing adhered surface-micromachined polycrystalline silicon (polysilicon) structures using pulsed lasers has been developed, experimentally investigated, and modeled. Advantages of using lasers to repair adhered microfabricated structures include the ease of implementation, the ability to apply the laser energy locally to the adhered region, and the non-contact nature of the process. The amount of thermal heating is typically less than 100 °C and does not damage polysilicon structures. Pulsed laser repair has been successful for undoped and n-doped polysilicon microcantilevers that had adhered to the substrate during chemical etching of the sacrificial layers and subsequent drying. This paper reviews experiments investigating the use of Ti:sapphire and Nd:YAG laser systems to repair failed polysilicon microcantilevers. The first experiments used a Ti:sapphire laser system, and later experiments using a Nd:YAG laser produced repair yields up to 100% for 10 μm wide cantilevers that were as long as 1 mm with laser fluences of 70 mJ/cm2. While the Ti:sapphire laser experiments indicate contributions from both non-thermal and thermal mechanisms for the repair process, the Nd:YAG laser experimental results are consistent with a thermal mechanism, specifically thermal expansion. A thermomechanical model is presented which enables the prediction of the laser operating conditions needed for laser repair. The model calculates the increase in strain energy due to heating a cantilever to a temperature higher than the substrate, and equilibrium crack lengths are determined by equating the adhesion energy and the strain energy release rate. The equilibrium crack lengths predicted by the model agree qualitatively and are within 15% of measured crack lengths. Increased adhesion energy decreases the equilibrium crack lengths for both single and multiple pulse laser operations. The laser repair process is predicted to increase the free lengths of micromachined polysilicon cantilevers from that achieved immediately after the drying process.

A Study on Thermal Stability of CoSi2 Employing Novel Fine-Grained Polycrystalline Silicon/CoSi2/Si (001) System

The cobalt silicide/fine-grained polycrystalline silicon (poly-Si) structure has been employed as gate electrodes in silicon-based very large-scale integration circuits. We have constructed a novel fine-grained poly-Si/cobalt silicide/silicon (001) structure to investigate the thermal stability of cobalt silicide at elevated temperatures. The dissociated cobalt atoms are observed to diffuse from fine-grained poly-Si/cobalt silicide and cobalt silicide/silicon (001) interfaces into the fine-grained poly-Si layer through poly-Si grain boundaries and the bulk cobalt silicide layer. The dissociated Si atoms at the cobalt silicide/silicon (001) interface are observed to grow epitaxilly on the silicon (001) substrate. This observation is consistent with previous results for circuits that employed amorphous Si instead of fine-grained poly-Si.

Structure and transport properties of p-type polycrystalline silicon fabricated from fluorinated source gas

P-type polycrystalline silicon (poly-Si) thin films are fabricated from SiF 4, H 2 and SiH 4 gas mixtures by plasma-enhanced chemical vapor deposition using in situ doping of B 2H 6 or BF 3. Relationships between deposition condition, film structure and transport properties are focused on. In addition, ionization efficiency is discussed in connection with deposition condition and with B and F concentrations. When poly-Si is doped with B, crystal fractions are lower for 300°C-grown films than for 200°C-grown films. High B doping results in (220) preferential orientation even when no fluorinated gas is used in the gas source. These trends are very similar among films prepared using different doping sources, i.e. B 2H 6 and BF 3. When SiF 4 is used, the B ionization efficiency is very low, ∼10%. It is improved to ∼50% by removing SiF 4 from the gas source. This low ionization efficiency shows good correlation with atomic concentration ratio of B/F in the poly-Si films, suggesting that some high incorporation of F compared to B inhibits the ionization of B. Using SiH 4, H 2 and BF 3 without SiF 4, highly (220) oriented, large grain poly-Si is obtained and it exhibits large mobility of 3.7 cm 2/Vs.

A Novel Poly-Si Solar Cell using Grain Boundary Etching Treatment and Transparent Conducting Oxide

ABSTRACTThis paper deals with a novel structure of polycrystalline silicon (poly-Si) solar cell for terrestrial applications. Grain boundary (GB) in poly-Si degrades a conversion efficiency of poly-Si solar cell. To reduce the GB side-effect, we investigated various parameters such as the preferential GB etch, etchtime, ITO electrode, heat treatment, and emitter layer effect. Among various preferential etchants such as Sirtl, Yang, Secco, and Schimmel, a Schimmel etchant illustrated an excellent preferential etching property. We observed a 10μm deep trench along grain boundaries and randomly textured grain surface with pyramid structure. We used rf magnetron sputter grown tin doped indium oxide (ITO) film as a top contact metal. ITO films showed a resistivity of 1.14 ×10-4 Ω-cm and transmittance of 90.5% for the wavelength of 594 nm. ITO films served as a top electrode as well as an effective AR coating layer. ITO film properties were strongly influenced by the preparation temperatures. Substrate temperature of 400oC gave the highest conversion efficiency of poly-Si solar cell. With well-fabricated poly-Si solar cells, we were able to achieve conversion efficiency as high as 16% at the input power of 20 mW/cm2.

Low-temperature wafer-level transfer bonding

In this paper, we present a new wafer-level transfer bonding technology. The technology can be used to transfer devices or films from one substrate wafer (sacrificial device wafer) to another substrate wafer (target wafer). The transfer bonding technology includes only low-temperature processes; thus, it is compatible with integrated circuits. The process flow consists of low-temperature adhesive bonding followed by sacrificially thinning of the device wafer. The transferred devices/films can be electrically interconnected to the target wafer (e.g., a CMOS wafer) if required. We present three example devices for which we have used the transfer bonding technology. The examples include two polycrystalline silicon structures and a test device for temperature coefficient of resistance measurements of thin-film materials. One of the main advantages of the new transfer bonding technology is that transducers and integrated circuits can be independently processed and optimized on different wafers before integrating the transducers on the integrated circuit wafer. Thus, the transducers can be made of, e.g., monocrystalline silicon or other high-temperature annealed, high-performance materials. Wafer-level transfer bonding can be a competitive alternative to flip-chip bonding, especially for thin-film devices with small feature sizes and when small electrical interconnections (<3/spl times/3 /spl mu/m/sup 2/) between the devices and the target wafer are required.